It is not clear exactly how many of the women have been separated from their children.

Aisha's children, aged 10 and six, are being looked after by her sister in Kilburn, north-west London, and she can only see them once a fortnight when they come and visit her, she added.

The 29-year-old Nigerian came to the UK in 1999 and was imprisoned in 2004 for using false documents to enrol at a university in London, she said.

She served six months behind bars and on her release was taken to Yarl's Wood.

Lisa Nandy, policy adviser at the Children's Society, and Celia Clarke, director of Bail for Immigration Detainees, said in a joint statement: ''We are very concerned about the reports we have received from detainees in Yarl's Wood Immigration Removal Centre that there has been a disturbance that has resulted in the police being called.

''We understand that a number of women are protesting against their enforced separation from their children and are extremely distressed.

''We urge the UK Border Agency (UKBA) to ensure the safety of all the women involved in this protest. We share their concerns about the impact of separating a child from their mother.''

The UKBA said the detainees would be integrated back into the centre at the earliest opportunity.

David Wood, strategic director of the UKBA's criminality and detention group, said: ''Around 40 women at Yarl's Wood Immigration Removal Centre are protesting against their detention and have been separated from other detainees while staff try to resolve concerns raised by these individuals.

''The well-being of detainees is of paramount concern to the UK Border Agency, which is why healthcare staff are at the scene to monitor developments.

''All detainees are treated with dignity and respect, with access to legal advice and healthcare facilities. We only remove those who both the UKBA and the independent courts deem to have no legal right to be here.''

Today's incident was contained within the compound, Bedfordshire Police said.

A spokeswoman said: ''Bedfordshire Police were called by staff at Yarl's Wood immigration detention centre at 1.30pm following reports of minor disorder involving a small number of detainees.

''The police helicopter flew over the area at lunchtime for assessment purposes but has since left.

''As of 2pm today all detainees were described as calm and talking with Yarl's Wood staff to address their concerns.

''Police officers, including dog units, are on standby outside the perimeter fence of the compound and will not be involved unless requested directly.''

No one is thought to have been injured.

Women behind the Wire is calling for an end to the detention of children and their mothers, rape survivors and other torture victims; an end to the detention of physically or mentally sick people and pregnant women for long periods of time; enough time and resources for residents who need to present their cases; access to appropriate medical treatment and care; access to edible and well-cooked food; and phones with good mobile connections including camera and recording facilities to back up cases.

The group said it also wants to stop the forceful removal and deportation of detainees and detention for asylum seekers and torture victims.

Yarl's Wood opened in 2001 and has become the UK's main removal centre for women and families.